Push dataset infos.json to Hub
When doing `push_to_hub`, the feature types are lost (see issue https://github.com/huggingface/datasets/issues/3394).
This PR fixes this by also pushing a `dataset_infos.json` file to the Hub, that stores the feature types.

Other minor changes:
- renamed the `___` separator to `--`, since `--` is now disallowed in a name in the back-end.

I tested this feature with datasets like conll2003 that has feature types like `ClassLabel` that were previously lost.

Close https://github.com/huggingface/datasets/issues/3394

I would like to include this in today's release (though not mandatory), so feel free to comment/suggest changes